1. Technical Field
The present invention relates to a fixing device and an image forming apparatus including the same.
2. Related Art
Conventionally, an image forming apparatus such as a copier, a printer, and the like, employs a fixing device employing electromagnetic induction, which is both fast and energy-efficient. Such a fixing device is comprised of a magnetic flux generator formed of an excitation coil and a center core, which generates a magnetic field, and a fixing roller, for example. An electromagnetic induction heat generation layer in the fixing roller is heated by the magnetic field thus generated. The fixing device described above can directly heat the heat generation layer, which has the advantage of allowing the thermal capacity of the fixing device to be made smaller than that of a halogen heater, for example, thereby providing high thermal efficiency and high-speed heating.
In fixing images on recording media of different widths, heat distribution varies laterally across the fixing member depending on the width of the recording media, resulting in an uneven temperature distribution across the fixing member. For example, when recording media with a small width are printed and fixed, because heat is absorbed from the fixing device by the recording media in the area through which the sheet passes (hereinafter also referred to as the sheet passing area), the temperature of that section of the fixing roller corresponding to the sheet passing area decreases compared to other areas. When particularly small-size recording media are continuously printed, heat absorption becomes acute.
As such, in a state in which the fixing temperature increases at lateral ends in an axial direction of the fixing member, when toner images on the recording media having a large width are fixed, hot offset occurs at a portion where the temperature has risen remarkably. Further, when the temperature at the axially lateral ends exceeds an upper temperature limit, the fixing member may be damaged due to the excessive heat.
It is possible to control the fixing temperature of an entire area in the axial direction based on the temperature at the axially lateral ends; however, because the heat in the sheet passing area is continuously absorbed from the fixing member, the necessary temperature for fixing cannot be maintained and the temperature decreases. As such, when toner images on the recording media are fixed in a state in which the fixing temperature decreases, cold offset occurs at a portion where the temperature has decreased.
There is an approach involving mounting a demagnetizing coil to form a magnetic field in a direction reverse to that of the excitation coil, on the excitation coil, so that the heating area of the fixing roller is adjusted by the demagnetizing effect of the demagnetizing coil.
JP-2008-139475-A discloses, as a first example, a fixing device employing electromagnetic induction in which, as illustrated in FIG. 16, an excitation coil 201 to form the magnetic field and a first demagnetizing coil 2100 and a second demagnetizing coil 2200 both to partially decrease the magnetic field overlap.
In addition, JP-2008-040176-A discloses, as a second example, a fixing device that includes an excitation coil to induction-heat a fixing roller; a demagnetizing coil to generate magnetic flux in a direction to erase a magnetic flux generated by the excitation coil, and a power supplying section to supply alternating current to the excitation coil and which is not connected to the demagnetizing coil electrically.
The fixing device in the first example is configured to switch ON and OFF each demagnetizing coil corresponding to the sheet size, so that different varieties of sheets can be handled. Accordingly, even though the small-sized sheets are continuously printed, an excessive rise of the temperature of the heat generator in the non-sheet passing area can be prevented. In addition, because the demagnetizing coils overlap, a seam between the demagnetizing areas is eliminated, thereby preventing an excessive temperature increase of the heat generator.
Further, the fixing device in the second example is configured such that the excitation coil and the demagnetizing coil are not connected electrically, thereby securely and effectively preventing an excessive temperature increase of the heat generator in the non-sheet passing area even though the small-sized sheets are continuously printed. The excitation coil can be disposed opposite the fixing roller via the demagnetizing coil, which is more effective in preventing temperature increase in the non-sheet passing area.
On the other hand, both of the above-described fixing devices include a demagnetizing coil that first detects the temperature increase in the non-sheet passing area at both lateral ends and operates responsive to the detection result. As a result, even though the printing condition of the first print is set for a small-size sheet, the entire fixing roller needs to be heated, resulting in heating areas not required for the first print.
Accordingly, although generally successful in preventing excessive temperature increase at both lateral ends of the heat generator, the above-described demagnetizing coils have non-optimal heating efficiency, so that a predetermined standby time is required until the fixing device becomes ready for fixing.